It is well known to log or record certain electrical characteristics of earth formations surrounding a well borehole as a function of depth in order to determine the location and extent of oil-bearing strata and to quantify the amount of oil present in such strata. A log of formation resistivity versus depth may indicate the presence of hydrocarbons, since hydrocarbon-bearing formations typically exhibit a higher resistivity than formations containing mostly salt water. There are only three material parameters which affect an electromagnetic wave. They are conductivity (resistivity), magnetic permeability and dielectric constant. Conductivity provides an indication of the energy absorbing characteristics of the medium, while magnetic permeability and dielectric constant give a measure of the energy storing capacity of a material. The magnetic permeability of most earth materials is the same and is equal to the magnetic permeability of free space. It is therefore of very little use in electrical logging techniques.
It is well known that conductivity or resistivity (which is the reciprocal of the conductivity) has wide variation in value for earth materials and strongly affects electromagnetic waves. A propagating electromagnetic wave has two fundamental characteristics, amplitude and phase. By comparing the amplitude and phase of an electromagnetic wave as it passes receivers, propagation characteristics due to formation may be studied. Measurement of these two characteristics (or equivalently, the wave travel time and attenuation) may be used to determine the dielectric constant and the resistivity of the media through which the wave is propagated.
Dielectric oil well logging is aimed at determining the water saturation and the water salinity in the formation from measurements of the dielectric constant and the resistivity of the formation. A number of criteria enter into the selection of the frequency of the electromagnetic wave used to probe the formation. Depending on these criteria, service companies have fielded a number of different logging tools at different frequencies. However, no one tool in the prior art is capable of probing a formation over a broad band of frequencies. It is therefore advantageous to extend the frequency range to include the function of the dielectric and the resistivity (induction) logging tools.
The objective of dielectric logging is to measure the dielectric constant and the conductivity of earth formations at a specific frequency, and deduce therefrom the water saturation and the salinity of this water. It has been found that logging at various frequency domains has various specific advantages. For example, logging at relatively high frequencies, near 1000 MHz, one probes a depth of only a few inches into the formation, so that here one measures essentially the properties of the invaded zone (i.e., the zone where the formation fluid has been displaced by the fluid filtrate from the mud in the borehole). On the other hand, logging at relatively low frequencies, near 20 MHz, allows one to probe much deeper into the formation (.about. a few feet), yielding perhaps information on the virgin formation. Logging at intermediate frequencies causes one to probe intermediate depths. The low frequency tools are of the centralized mandrel type, since at these frequencies the loss of energy in the mud annulus between the tool and the formation can be tolerated. At high frequencies, however, this loss would be prohibitive, and hence the high frequency tools must necessarily be of the pad type, where the pad directly contacts the formation or borehole wall.
The development of tools at these many different frequencies shows that it is advantageous to create a single tool capable of logging:
(i) at a number of discrete frequencies PA1 (ii) over a continuous range of frequencies employing the swept frequency technique, and PA1 (iii) in the time-domain, resulting - upon inversion - in the same information as in (ii) above. PA1 (i) it would yield the profile of water saturation and water salinity with distance from the wellbore, and PA1 (ii) since a large amount of information would be gathered, this makes it possible to deduce, from the dielectric log alone, the porosity, in addition to the water saturation, and the water salinity.
The main advantages of such a tool would be:
U.S. Pat. No. 4,899,894, issued to Katahara et al., describes an acoustic logging tool, and does not discuss electromagnetic logging. It uses a plurality of acoustic frequencies only to accommodate a wide variety of formations having different preferred logging frequencies, and does not teach a method of broadband logging over each formation.
U.S Pat. No. 3,982,176, issued to Meador, discloses a conventional induction log (20 KHz) and a conventional dielectric log (16 MHz) in single tool, using different antennas (coils) for the different frequencies. This is a mandrel type tool, that hangs freely in a wellbore with an annulus of mud around it. There is no mention of broadband logging.
U.S. Pat. No. 4,451,789, also issued to Meador, discloses a method of depth-probing the formation by changing the transmitter-receiver spacing. Frequency variation is not discussed. Although Meador uses three frequencies, they are very closely spaced, and are not used for the purpose of broadband logging. Meador simply uses slightly different frequencies as a way of labeling the transmitter-receiver pairs It is therefore essentially a single frequency tool. It is also a mandrel type tool, which hangs freely in a wellbore.
U.S. Pat. No. 4,774,471, issued to Sims et al., discloses a single-frequency (between 10 MHz and 200 MHz) mandrel type tool Sims does not discuss how their antenna can accommodate this frequency range, or whether different antennas are used for different frequencies. This patent only uses a wider range of frequencies within the operation of an earlier tool. The patent does teach that broadband dielectric logging is useful, however.
The largest hurdle to developing a broadband dielectric logging tool has been the lack of a suitable broadband antenna that can couple electromagnetic energy to and from a formation, and that is compact enough to fit within the confines of a logging tool.
An additional advantage is that a dielectric measuring apparatus can be applied to the field of medical technology. For example, U.S. Pat. No. 4,240,445 issued to Iskander et al. teaches a method of coupling electromagnetic energy into a material such as tissue, to measure water content. Measuring lung water content is an especially useful application. However, Iskander's device is so large that only a few antennas can be place on the chest, and the antenna cannot be described as a point source. Also, the electric field vanishes at some distance from the antenna, as the electric fields in the two parallel slots are oppositely directed. Furthermore, a resistor is included in the antenna, which dissipates much of the electromagnetic energy in the antenna itself. Additional prior work includes M. F. Iskander and C. H. Durney (1980): "Electromagnetic Techniques for Medical Diagnosis: A Review", Proceedings of IEEE, vol. 68, no. 1. and M. F. Iskander et al (1982): "Two-dimensional Technique to Calculate the EM Power Deposition Pattern in the Human Body", Journal of Microwave Power, vol. 17, no. 3.
The prior work is limited in the attempts at broadband logging (or measuring) in that no suitable single antenna element has been designed which can couple electromagnetic energy into a material, whether it be a geologic formation or tissue, over a broad range of frequencies, that is also sufficiently compact and is capable of handling high power levels. There is therefore a need for a device and a method for use in such broadband logging/measuring/heating applications.